NKT cells express both a unique T cell receptor and NK marker and may be subdivided according to their cytokine profile and activation markers. NKT cells have been demonstrated to be important in the protection against infectious pathogens, immune surveillance of tumors and induction and control of autoimmunity. It is well known that the glycolipid alpha-galactosylceramide (alpha-GalCer) is presented by CD1d molecules on antigen presenting cells and potently activate NKT cells. However, the study of costimulation in antigen specific activation of NKT cells is still in its infancy. NKT cells express CD28 and blockade of this pathway leads to a decrease in IFN-gamma and IL-4 after stimulation. These results demonstrate the role of a positive costimulator in the activation of NKT cells. Recently, the number of T cell costimulatory family members has increased with some having positive or negative functions. The role of the negative costimulatory molecules such as the PD-1 pathway in NKT cell function has not been investigated. Preliminarily data shows that NKT cells express PD-1 and PD-L1 and leads us to hypothesize that the PD-1 pathway is involved in negatively regulating NKT cells. This hypothesis raises several key questions that will be addressed in this project 1) Do the PD-1 ligands have unique or overlapping roles in inhibiting NKT function 2) Are dendritic cells or another cell type the focus of the downregulating response of NKT cells? 3) Does blockade of the PD-1 pathway on NKT cells lead to a breakdown in tolerance. Therefore, in Aim 1 the development and function of NKT cells in PD-L1 deficient mice and PD-1 transgenic mice will be examined. In Aim 2, we will investigate the cross talk between dendritic cells and NKT cells lacking the PD-1 ligands. The success of the activation of NKT cells with alpha-GalCer in murine anti-tumor model has lead to a phase 1 study with a-GalCer in advanced patients with solid tumors. Our studies will give key insights into the role of PD-1 interactions in the inhibitory effects of NKT cells and may provide important targets for therapeutic interventions in tumor immunity, autoimmunity and transplantation. [unreadable] [unreadable] [unreadable]